1. Field of the Art
The present invention generally relates to acoustic devices, and more specifically to a planar speaker driver.
2. Description of the Related Art
Planar (planar-magnetic, ribbon, thin film drivers) drivers have always been praised for exceptional sound quality associated with their unique acoustic attributes. This invention describes a wide-band planar transducer with high sensitivity, extended lower frequency operating band, higher power handling and low distortion.
FIG. 1 illustrates a cross-section showing a basic construction of a typical planar-magnetic transducer. A common type of such transducer incorporates a diaphragm 1 with areas of multiple electrical conductors 2. A diaphragm 1 is clamped in a frame 5 and is positioned between two rows of magnet bars 3. Magnets are sequentially located on top and bottom metal plates 4 with spaced areas 7 between the magnets. Holes 6 in metal plates 4 correspond to the spaced areas 7 between magnets 3, acoustically connecting diaphragm 1 with outside media.
Magnets 3 are magnetized in a direction perpendicular to metal plate 4 so that a magnet from one side of a diaphragm and the opposite magnet from the other side of diaphragm are facing diaphragm and each other with the same magnetic poles (S or N). Each adjacent magnet bar that is located on the same side of the diaphragm has the opposite direction of magnetization, thus each following magnet faces the diaphragm with the opposite magnetic pole, following the sequence N,S,N,S,N and so on. Magnetic field created by the magnet arrangement has the magnetic flux vector B in a plane of the diaphragm across the lines of conductors.
When an electrical signal is applied to the diaphragm, the current that flows through conductors interacts with the magnetic field and resulting electromotive force makes the diaphragm vibrate in the direction perpendicular its plane. Vibrating, the diaphragm 1 radiates sound waves that emanate through the openings 7 between magnets 3 and holes 6 in metal plates 4 in both directions from the diaphragm 1. Different acoustical loading conditions may be applied to the design such as using a metal plate 4 with variations in the holes 6 (e.g., slots, or solid regions) or attaching an enclosure form one side of a transducer.
The use of rear earth magnetic materials such as NdFeB (Neodymium) that has become the magnet material of choice in transducers recent years, allows significant reduction of size and efficiency improvement of transducer designs. As a result such designs can provide very high quality sound with minimal front to back space required, thus allowing building of “flat” panel planar loudspeakers for many critical applications.
Among performance limitations traditionally associated with planar drivers are limited low frequency extension and limited dynamic range at those frequencies. Both of these issues are mostly related to two aspects of driver design and operation: maximum diaphragm excursion capability and vibration behavior of the diaphragm within the operating range.
In order to extend effective frequency range of such design in a region of lower frequencies, a transducer has to have significant radiating area. However, a larger diaphragm has much less vibration control and generates significant modal vibrations due to insufficient mechanical losses in diaphragm substrate, usually plastic film. These pronounced vibrations at diaphragm resonance frequencies lead to response irregularities and parasitic noises at lower frequencies that are very often encountered in planar transducers.
Many designs use coating of the diaphragm with dampening materials and/or corrugation over the whole diaphragm area. Both of these methods have negative effects. A coating leads to higher mass and efficiency losses. Corrugation of the entire diaphragm increases the effective thickness of the diaphragm where active conductions are located and thus limits maximum excursion of the diaphragm. Additionally the corrugation of diaphragm in the area of active conductors that are made of very thin metal foil can introduce internal stresses in the conductor and/or in the bond between polymer film and the foil conductor. Under high thermal and mechanical stress due to vibrations the internal stresses can then lead to premature de-lamination or cracks in the conductors.